|
|
|
Challenges Come to the Surface in Direct Chill Casting
01-17-03
Balancing productivity, safety, and surface quality in the production of direct chill cast ingot has been the focus of much development work in the industry. Through combined metallurgical understanding and process innovation along with equipment and automation advances, the ability to make increasingly larger ingots from a range of aluminum alloys with reduced scalping requirements and higher recovery is being realized. What's going on at the ingot surface is critical.
The process invented in the 1930's for direct chill casting remains fundamentally the same. Molten aluminum is poured into a mold defining the cross sectional dimensions of the ingot, but having a mold length dimension of only a few inches. To start the ingot cast, another moving component called the bottom block is brought into contact with the mold and then gradually lowered as a solidified ingot shell is formed. The forming ingot actually still contains molten alloy in the center as it leaves the mold, contained by the solidified shell. Considering that this process is employed for ingots with rectangular cross sections on the order of 15-25 inches x 50-100 inches or greater and weights up to 75,000 pounds, it is a feat of engineering and scientific understanding combined that it is done repeatedly as a regular production method.
While there are many aspects of the process that must be considered, the details of the formation of the initial ingot "skin" at the mold-metal interface represent some of the most crucial. There are competing needs here. On the one hand, a sufficiently strong and fracture-resistant ingot shell must be formed to support and contain the unsolidified interior, which is favored by longer mold contact length. This is especially important during the start-up phase of the cast, where issues such as butt curl can influence the propensity for hot cracking. This must be balanced with the desire to minimize the indirect cooling and mold contact to reduce the formation of surface segregation and the so-called "shell zone" microstructure, with the goal of reducing or eliminating scalping and edge trim requirements during subsequent wrought processing.
Managing the heat transfer and other mechanical processes occurring in the mold, especially at the early stages of solidification, has been the focus of industry as well as academic attention. It is during this time that the surface structure of the ingot is established, and it is this initial structure that critically defines the downstream performance of the cast product.
One set of approaches has involved techniques to control and minimize mold contact. In processes used for casting of cylindrical billets for extrusion processing, heat extraction in the mold has been reduced through continuous feeding of gas combined with lubricant into the gap between the melt and the mold wall. Known by trade names such as AirSlip™, and Airsol Veil™, these methods, along with hot-top molds allowing uniformity of metal flow into the mold, enable multiple ingots to be cast with good surface and edge structure quality. A good summary of issues for extrusion ingot casting can be found in the article by Jarrett, et al. in the ET2000 Proceedings (available from The Aluminum Association (www.aluminum.org) or the Aluminum Extruders Council, www.aec.org).
Similar concepts are applied to the production of rectangular rolling ingots. Electromagnetic casting (EMC) uses a magnetic field to separate the melt and the mold so that the ingot edge and mold wall do not touch. An alternative process involves a low head composite mold consisting of porous graphite through which lubricant is transported to the mold surface. Again, by controlling heat extraction in the indirect cooling zone, surface and edge quality improvements over conventional DC casting, and hence reduced scalping and edge trim requirements, have been realized. A good although brief discussion of these technologies can be found in a recent overview paper by Schneider in Light Metals 2002, published by TMS (www.tms.org).
A completely different approach to controlling ingot surface formation involves
modification of the mold surface itself. This work at Cornell University's Materials
Process Design and Control Laboratory, sponsored by Alcoa and the Department of
Energy's Office of Industrial Technologies, involves producing mold surface topographies
consisting of unidirectional and bi-directional groove textures to control heat
extraction and surface structure. This program combines analytical modeling of
the surface formation process along with experimental studies to validate the
models. For further information on this work, contact Professor Zabaras at www.mae.cornell.edu/zabaras/People/zabarasCV/Zabaras.html.
Another aspect of heat extraction relates to the cooling water, in particular
its consistency in terms of additives (intentional or otherwise), temperature,
and pH. Variations that can occur can greatly affect the rate of heat removal,
and hence the surface development during initial solidification. In some cases
intentional additions such as CO2 or injected air are used to control the film
boiling to nucleate boiling transition in commercial practice. To increase the
theoretical understanding of cooling water performance, work is underway at Idaho
National Engineering and Environmental Laboratory and the University of California-Berkeley
in cooperation with Alcoa, Inc. and supported by the DOE Office of Industrial
Technologies. For more information, see http://www.eere.energy.gov/industry/aluminum/pdfs/coolantcharacteristics.pdf
A discussion about advances in ingot casting for aluminum would be incomplete without reference to the improvements achieved in automating the casting processes. An intriguing article entitled "Aluminum: Approaching the New Millennium", (JOM, February 1999, pp. 29-42) describes the transformation as follows: "The pails, yard sticks, and stop watches of yesteryear have yielded to computer-integrated casting process controls that include accurate measurement and control of water flow rates, water temperature, metal levels (controlled to within +/-1 mm), drop rates, CO2 and air injection ramping, metal temperatures, in-line systems operation, electrical systems (in the case of electromagnetic casting), lubrication, air flow for air-injected molds, and lengths. These systems include automatic shutdown provisions." Certainly, as new developments and improvements in the process are developed, these will be added to the control architecture as well.
In summary, while much of the current work in the area of ingot casting focuses on process improvement and refinement as opposed to invention, the increasing efforts to improve understanding of the process at a fundamental level, especially in the surface and near-surface regions, should reap future benefits in terms of productivity, quality, and safety.
Article courtesy of Secat, Inc. - Research Resource for the Aluminum Industry
www.secat.net
|
|
|
